In general, permanent waving of hair is effected by chemically cleaving the disulfide bonds that occur naturally in hair with a reducing agent, optionally rinsing, and forming new disulfide bonds with an oxidizing agent while the hair is under tension. Permanent waving can be used to either increase or decrease the amount of curl in hair.
Sulfur containing agents are commonly used to reduce the disulfide bonds in hair. These agents include organic thiol containing compounds and inorganic sulfur containing compounds and are used either individually or in combination. Examples of organic thiols used in waving compositions are compounds such as cysteine, cysteamine, N-substituted cysteamines, alkyl substituted mercaptoacetamides, dimercaptoadipic acid, thioglycerol, thiolactic acid, thioglycolic acid or its salts, (e.g., a thioglycolate), monothioglycolic acid esters such as diol esters of thioglycolic acid, glyceryl monothioglycolate, thiocholine or its salts, amino thiols, and thiols attached to low molecular weight polymers. Examples of inorganic reducing agents used in waving compositions are compounds such as sodium hyposulfite and ammonium or sodium bisulfite.
One of the most obvious and negative characteristics of a permanent wave is the strong chemical odor. For example, in many commercially available wave kits an unpleasant odor is produced by the thiol reducing agent as well as from ammonia. In addition to the odor produced during the treatment process, hair is often left with a "sulfur" smell that can take days or weeks to dissipate. The cause of the odor may be due to the sulfur containing reducing agent itself or a degradation product.
Many permanent waves are based on thioglycolic acid (TGA). This reagent shows good performance, low cost, low damage and good toxicology. The major disadvantages of TGA is the unpleasant odor and high pH at which is must be used, which is considered damaging. Thiolactic acid is less efficient than TGA but is milder on the epidermis. It is sometimes used in combination with TGA. Glyceryl monothioglycolate (GMT) has been used for a number of years in "acid waves", which are formulated at a pH of about 7-8. A low pH formulation is possible because of the lower pKa of this thiol (7.8) compared to thioglycolic acid (10.4). These waves are considered softer and less damaging than those obtained from TGA, presumably due to the lower pH. The disadvantage of this reagent is that it causes skin irritation in some people. In addition, the post-perm odor is more pronounced from GMT (glyceryl monothioglycolate) acid waves, than from ATG (ammonium thioglycolate) alkaline waves.
Cysteamine has a pKa of 8.6, so it may be formulated at a pH near 8. The major disadvantage of this reagent is the post-penn odor which has a different characteristic than thioglycolic acid. This odor has been likened to burnt popcorn or corn chips. Another agent, cysteine, which has been marketed in Japan as a natural reagent, produces weaker curls than TGA and is an irritant. Another problem with cysteine is that its corresponding disulfide by-product forms a water-insoluble precipitate which looks like flakes of dandruff. It has a high pKa (10.8) and thus, as with TGA, ester derivatives are used to lower the pKa of the thiol and allow for lower pH formulations.
Thiols, also known as mercaptans, are known to degrade to hydrogen sulfide via both radical and nucleophilic mechanisms. In fact, gas chromatography has been used to detect ppm levels of hydrogen sulfide in the headspace of ammonium thioglycolate waving compositions' and of methyl mercaptan in the headspace of a vessel containing hair permed with ammonium thioglycolate..sup.2 Post-penn odor is also attributed to the disulfide byproduct that is formed from the reaction of K--S--S--K+2 R--S--H.fwdarw.R--S--S--R+2 K--S--H, where K= hair keratin..sup.3 This explanation would account for the greater post-penn odor from GMT since this disulfide will be less water soluble than the disulfide from ATG and, thus, less effectively washed away. Other evidence suggests that the thiols become physically or chemically bound to the hair during the penning process..sup.4
The odor released by the sulfur-based waving compositions is a significant drawback and many unsuccessful attempts have been made to mask the odor with perfumes. In addition, attempts have been made to remove the malodorous compounds. For example, U.S. Pat. No. 5,184,630 describes an aqueous suspension of siliceous crystalline molecular sieves to remove post-perm odor on hair. U.S. Pat. No. 4,560,554 describes the incorporation of between 0.0001 and 5 weight percent of a ketone, such as dibenzyl ketone or 2-hydroxyl-1,4-napthoquinone, to the hydrogen peroxide neutralizer to eliminate mercaptan odor on hair. U.S. Pat. No. 4,547,365 describes the incorporation of a ketone and a cyclodextrin in the mercapto waving composition to lower odor. U.S. Pat. No. 5,843,420 describes the addition of a powdered, odor removing absorbing agent which is insoluble in the thiol containing waving composition formulation. EP 246,090 describes the reduction of perm odor by using a cyclodextrin in conjunction with a cationic polymeric material together in a thioglycolate waving composition or contained in a post-perm shampoo. U.S. Pat. No. 5,715,845 describes the use of a manganese salt prior to, or during, the reduction step to lower the post-perm odor. WO 90/00385 describes the use of between 0.05 and 5 weight percent of peroxydisulfate with a hydrogen peroxide neutralizer to eliminate post-penn odor from hair due to higher oxidizing ability of persulfate. EP 302,265 describes the incorporation of isosorbide dimethyl ether into a thiol containing wave composition. JP 10 101,534 describes an antibacterial substance derived from dry distillation of higher plants to remove malodor.
In addition, work has focused on improving cysteamine post-perm odor. Researchers have found that odor can result from reaction between hair aldehydes and cysteamine and have employed chemicals to competitively react with either of these components. One theory is that post-perm odor results from odoriferous residues made up of thiazolidine compounds that are formed upon reaction of the aldehydes in the hair with the thiol reducing agent..sup.5 These residues remain in the hair, either because they are chemically bound or trapped within the hair matrix. Over time the residues are released and the odor is noted..sup.6 Use of polyhydric phenols, such as resorcinol or its derivatives, reduce the post-perm odor because they react with hair aldehydes thus blocking these sights from the reducing agent (U.S. Pat. No. 5,554,364). These polyhydric phenols may be used before, during or after treatment with reducing solution, or after neutralization since they are said to be substantive to hair; they are particularly useful when cysteamine is the reducing agent
A related approach involves treating hair with an aldehyde solution either before treatment with cysteamine or after, but before neutralization (U.S. Pat. No. 5,554,363 and 5,554,363). These aldehydes compete with the hair aldehydes for the residual reducing agent and produce a different, more pleasant odor. Benzaldehyde or methyl hydrocinnamic aldehyde is preferred since they react faster than hair aldehydes. Presumably the reaction of thiols with aldehydes is slower than their reaction with hair disulfides. A similar patent, U.S. Pat. No. 5,540,910, describes a process in which specific aliphatic aldehydes are applied to hair after the reduction step, and either before or after neutralization (U.S. Pat. No. 5,540,910). The use of thioglycolic acid after cysteamine is reported to prevent odor (WO 9306817)
However, none of these methods eliminate or reduce the perm-associated malodor to acceptable levels. According to the present invention, disulfides are used to reduce perm-associated malodor during permanent waving, and/or to reduce malodor on hair after permanent waving. Although the use of disulfides with thiol reducing agents is described in the prior art as a method for providing "stop-action" or "self-timing" waves, these disulfides are used at concentrations much higher relative to thiol than in the present invention. The ratio of disulfide to thiol is typically 1:20 to 1:5 in these "stop action" products. A self-neutralizing wave at even higher disulfide to thiol ratio is described in U.S. Pat. No. 3,768,490. The use of disulfides as a neutralizer and as an additive to hair waving compositions has also been described. For example, the use of glutathione disulfide to crosslink hair thiols is described in U.S. Pat. No. 4,812,307, and the use of pantethine to improve waving efficiency in permanent waving is described in U.S. Pat. No. 4,851,215. However, none of the prior art patents describe the odor reducing properties of disulfides which are taught in the present invention.